Background: Music festivals are known to attract a high proportion of drug users. Methods: Using a survey of 1,772 visitors at the Roskilde Festival in Denmark, we assessed substance use at the festival, the incidence of use of substances among never-users and the incidence of use among lifetime users who had not used a substance in the past 12 months. Results: New onset of tobacco use was reported by 9.2% of never-smokers, and new onset of cannabis use was reported by 9.3% of never-smokers of cannabis. Resumption of tobacco use was reported by 24% of past year abstainers, and resumption of cannabis use was reported by 30% of past year abstainers. New onset of other types of substances was reported by less than 0.5% of subjects, but among past year abstainers, 5-10% reported resumption of amphetamine, ketamine, MDMA and cocaine use. New onset smokers of cannabis were significantly younger than never-smokers. Conclusion: Music festivals such as the Roskilde Festival may be important arenas for the prevention of onset of tobacco and cannabis use and for a return to substance use.